1. Field of the Invention
The present invention relates to a synchronous motor control device for controlling a synchronous motor to carry out a power regenerative operation to return regenerative energy, which is generated during speed reduction of the synchronous motor, to the alternating-current power source side, and stop the synchronous motor when an alternating-current power source fails.
2. Description of Related Art
Conventionally, in order to handle regenerative energy, which is generated during speed reduction of a motor, a motor control device for controlling a motor to carry out power regenerative operation to return regenerative energy to the alternating-current power source side is used. In the case where such a power regenerative operation is carried out, it is not possible to return regenerative energy, which is generated during speed reduction of the motor, to the alternating-current power source side when the alternating-current power source fails, and therefore, an over-voltage (anomalous voltage) of a DC link part, which is connected in parallel to an inverter configured to supply alternating-current power to the motor, occurs. In order to avoid the over-voltage across the DC link part, a motor control device, in which a regenerative resistor or dynamic brake is provided to consume regenerative energy caused by the speed reduction of the motor when an alternating-current power source fails, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-243675 (JP10-243675A) and Japanese Unexamined Patent Publication (Kokai) No. 2002-369564 (JP2002-369564A).
However, in the case where a regenerative resistor or dynamic brake is provided in a motor control device, it is necessary to increase the resistance of the regenerative resistor or the capacity of the dynamic brake as the output of the motor increases, and therefore, there is such a disadvantage that the cost of the motor control device increases as the resistance of the regenerative resistor or the capacity of the dynamic brake increases.
On the other hand, an induction motor control device for controlling an induction motor so as to make it possible to handle regenerative energy caused by speed reduction of a motor when an alternating-current power source fails without providing a regenerative resistor or dynamic brake, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-51182 (JP5-51182A). In such an induction motor control device, the value of a q-axis current of the induction motor increases during the speed reduction of the induction motor when the alternating-current power source fails, by controlling the value of a d-axis current of the induction motor under the same torque condition, and the generation of the regenerative energy, which is returned to the alternating-current power source side, is prevented by setting the operative efficiency of the induction motor at the time of the driving of the induction motor during the speed reduction of the induction motor lower than the operative efficiency of the induction motor at the time of the driving of the induction motor except for the driving of the induction motor during the speed reduction of the induction motor. Further, in the induction motor control device described above, at the time of the driving of the induction motor except for the driving of the induction motor during the speed reduction of the induction motor when the alternating-current power source fails, the value of the d-axis current is set to a value larger than the value of the d-axis current, which is set during the speed reduction of the induction motor, in order to set the operative efficiency of the induction motor larger than the operative efficiency of the induction motor during the speed reduction of the induction motor.
However, in the case where a synchronous motor is controlled so as to carry out the power regenerative operation, the q-axis current contributes to generation of a torque, and therefore, if the value of the d-axis current of the synchronous motor is suppressed and the value of the q-axis current of the synchronous motor is increased during the speed reduction of the synchronous motor when the alternating-current power source fails, the regenerative energy increases, and therefore, there is a possibility of the issuance of an over-voltage alarm due to an over-voltage at the DC link part, which is connected in parallel to the inverter configured to supply alternating-current power to the synchronous motor. Further, in the case where the value of the d-axis current is set to a value larger than zero at the time of the driving of the synchronous motor except for the driving of the synchronous motor during the speed reduction of the synchronous motor when the alternating-current power source fails, power consumed by the synchronous motor increases and the operating efficiency of the synchronous motor reduces remarkably.